Colloidal semiconductor quantum dots (QDs) constitute a perfect material for ink-jet printable large area displays, photovoltaics, light-emitting diode, bio-imaging luminescent markers and many other applications. For this purpose, efficient light emission/ absorption and spectral tunability are necessary conditions. These are currently fulfilled by the direct bandgap materials. Si-QDs could offer the solution to major hurdles posed by these materials, namely, toxicity (e.g., Cd-, Pb-or As-based QDs), scarcity (e.g., QD with In, Se, Te) and/or instability. Here we show that by combining quantum confinement with dedicated surface engineering, the biggest drawback of Si-the indirect bandgap nature-can be overcome, and a 'direct bandgap' variety of Si-QDs is created. We demonstrate this transformation on chemically synthesized Si-QDs using state-of-the-art optical spectroscopy and theoretical modelling. The carbon surface termination gives rise to drastic modification in electron and hole wavefunctions and radiative transitions between the lowest excited states of electron and hole attain 'direct bandgap-like' (phonon-less) character. This results in efficient fast emission, tunable within the visible spectral range by QD size. These findings are fully justified within a tight-binding theoretical model. When the C surface termination is replaced by oxygen, the emission is converted into the well-known red luminescence, with microsecond decay and limited spectral tunability. In that way, the 'direct bandgap' Si-QDs convert into the 'traditional' indirect bandgap form, thoroughly investigated in the past.

We calculate the ground and excited electron and hole levels in spherical Si quantum dots inside SiO 2 in a multiband effective mass approximation. The Luttinger Hamiltonian is used for holes, and the strong anisotropy of the conduction electron effective mass in Si is taken into account. As the boundary conditions for the electron and hole wave functions, we use the continuity of the wave functions and the flux at the boundaries of the quantum dots.

We present a high-resolution photoluminescence study of Er-doped SiO 2 sensitized with Si nanocrystals ͑Si NCs͒. Emission bands originating from recombination of excitons confined in Si NCs, internal transitions within the 4f-electron core of Er 3+ ions, and a band centered at Ϸ 1200 nm have been identified. Their kinetics were investigated in detail. Based on these measurements, we present a comprehensive model for energy-transfer mechanisms responsible for light generation in this system. A unique picture of energy flow between the two subsystems was developed, yielding truly microscopic information on the sensitization effect and its limitations. In particular, we show that most of the Er 3+ ions available in the system are participating in the energy exchange. The long-standing problem of apparent loss of optical activity in the majority of Er dopants upon sensitization with Si NCs is clarified and assigned to the appearance of a very efficient energy exchange mechanism between Si NCs and Er 3+ ions. Application potential of SiO 2 : Er, sensitized by Si NCs, was discussed in view of the newly acquired microscopic insight.

The effect of quantum confinement on the direct bandgap of spherical Si nanocrystals has been modelled theoretically. We conclude that the energy of the direct bandgap at the $\Gamma$-point decreases with size reduction: quantum confinement enhances radiative recombination across the direct bandgap and introduces its "red" shift for smaller grains. We postulate to identify the frequently reported efficient blue emission (F-band) from Si nanocrystals with this zero-phonon recombination. In a dedicated experiment, we confirm the "red" shift of the F-band, supporting the proposed identification

We report results of time-resolved induced absorption (IA) spectroscopy on Si nanocrystals (Si NCs) embedded in a SiO 2 matrix. In line with theoretical modeling, the IA amplitude decreases with probing photon energy, however only until a certain threshold value. For larger photon energies, an increase of IA is observed. This unexpected behavior is interpreted in terms of the self-trapped exciton state whose formation in Si NCs was put forward some time ago based on theoretical considerations. Here, we present a direct experimental confirmation of this supposition. Silicon nanocrystals (NCs) 1 are frequently investigated for their interesting optical properties and a wide variety of potential applications in optoelectronics, 2-4 photovoltaics, [5][6][7] and the medical field. 8 In particular, the photoluminescence (PL) of Si NCs has been thoroughly characterized by experiment 9,10 and extensively modeled by theory using the ab initio approach 11 as well as the semiempirical methods: pseudopotential, 12 tight-binding, 13,14 and effective mass approximation. 15 In that way, opening of the (indirect) band gap and enhancement of the radiative rate of band-to-band recombination have been firmly established. For oxygenterminated Si NCs a peculiarity has been found: for smaller diameters d NC < 2.5 nm the blueshift in PL spectrum could not be observed, with PL energy stabilizing in the visible range. This has been explained in terms of formation of oxygenrelated defects at the surface of NCs, with levels appearing in the band gap and participating in the recombination of carriers. 16 Specific microscopic details of these defects are not known, but oxygen is well known to form electrically active defects in bulk Si. 17,18 Among other possibilities, formation of a self-trapped exciton state (STE) has been proposed. 19 Support for the existence of the STE state facilitating photon emission in small oxygen-terminated Si NCs was inferred only indirectly from steady-state PL experiments-predominantly from the aforementioned stabilization of the quantum confinement induced blueshift of PL and from the temperature dependence of PL intensity and lifetime. 20 It is fair to say that an experimental evidence directly confirming formation of the STE is still missing.In order to investigate formation and characteristics of the STE state, carrier dynamics directly after photoexcitation need to be investigated. This is best accomplished by means of ultrafast induced absorption (IA) and PL up-conversion spectroscopy. Past investigations revealed that carrier dynamics in Si NCs exhibits always a fast multiexponential decay. [21][22][23][24] This illustrates a variety of carrier relaxation pathways, with individual components assigned to trapping, 25 carrier-carrier scattering, Auger energy transfer between electrons and holes, 14 phonon-assisted cooling, and no-phonon radiative recombination. 10 In that landscape, formation of the STE state has been related to trapping to defects at the surface of Si NCs, competing with carrier cooling....

No abstract

Dynamics of hot carriers confined in Si nanocrystals is studied theoretically using atomistic tight binding approach. Radiative, Auger-like and phonon-assisted processes are considered. The Auger-like energy exchange between electrons and holes is found to be the fastest process in the system. However the energy relaxation of hot electron-hole pair is governed by the single optical phonon emission. For a considerable number of states in small nanocrystals single-phonon processes are ruled out by energy conservation law.

We report on investigations of optical carrier generation in silicon nanocrystals embedded in an SiO 2 matrix. Carrier relaxation and recombination processes are monitored by means of time-resolved induced absorption, using a conventional femtosecond pump-probe setup for samples containing different average sizes of nanocrystals (d NC = 2.5-5.5 nm). The electron-hole pairs generated by the pump pulse are probed by a second pulse over a broad spectral range (E probe = 0.95-1.35 or 1.6-3.25 eV), by which information on excited states is obtained. Under the same excitation conditions, we observe that the induced absorption intensity in the near-infrared range is a factor of ∼10 higher than in the visible range. To account for these observations, we model the spectral dependence of the induced absorption signal using an empirical sp 3 d 5 s * tight-binding technique, by which the spectrum can be well reproduced up to a certain threshold. For probe photon energies above this threshold (dependent on nanocrystal size), the induced absorption signal is found to feature a long-standing component, whereas the induced absorption signal for probe photon energies below this value vanishes within 0.5 ns. We explain this by self-trapping of excitons on surface-related states.